having 2 miners shouldn't improve performance at all. theoretically, the performance should be lower because the card has to switch between the 2 miners.
I would say it should be higher because cards don't "switch" - they wait for tasks to be thrown at it in numerous little chunks, and mining is a whole bunch of little tasks. A second instance could mop up some of the time the card spends waiting on a CPU to send it more units of work.